It looks like these dispenser sizes came from Defense Technology, one of the earliest companies selling OC. They have an explainer sheet in PDF form here.
The Mk.3, in the middle, is kinda chonky for pocket carry, and more at home on a duty belt (or maybe stashed by your front door). The one on the right is the Mk.6 size, which is about as big as you'd want to carry in a pants pocket, and only if your jeans aren't too snug. It takes a fairly roomy pocket.
On the left is the standard POM pocket/keychain sized unit, which sacrifices capacity for portability.
You'll note that Def-Tec's literature says that the Mk.6 is capable of "12-14 short bursts", while POM's advertising copy for its little pocket spritzer claims "up to 20+ half-second bursts". Yet the POM is a half-ounce canister while the Mk.6 has .68 ounces of payload. This is because in real life nobody gets a half-second spritz. It generally takes a good "One Mississippi" to paint a proper back-'n'-forth orange stripe across Sumdood's peepers.
I'll carry either the POM or the Sabre Red Mk.6. (And if you're ordering Mk.6 make sure it's not foam or gel, and that it doesn't have any goofy additives like CS, since the latter just makes decontam more of a pain without adding any benefits on the front end.)
